A typical electronic image sensor comprises a number of photodiodes or other photosensitive elements arranged in a two-dimensional array. These elements are also commonly referred to as picture elements or “pixels” and the corresponding array is referred to as a pixel array.
In order to allow such an image sensor to produce a color image, the image sensor is configured with color filter elements of a color filter array (CFA) arranged over respective ones of the pixels. One commonly used type of CFA pattern is the Bayer pattern, disclosed in U.S. Pat. No. 3,971,065, entitled “Color Imaging Array,” which is incorporated by reference herein. The Bayer CFA pattern provides each pixel with a color photoresponse exhibiting a predominant sensitivity to one of three designated portions of the visible spectrum. The three designated portions may be, for example, red, green and blue, or cyan, magenta and yellow. A given CFA pattern is generally characterized by a minimal repeating unit in the form of a subarray of contiguous pixels that acts as a basic building block for the pattern. Multiple copies of the minimal repeating unit are juxtaposed to form the complete pattern.
An image captured using an image sensor with a Bayer CFA pattern has only one color value at each pixel. Therefore, in order to produce a full color image, the missing color values at each pixel are interpolated from the color values of nearby pixels. Numerous such interpolation techniques are known in the art. See, for example, U.S. Pat. No. 5,652,621, entitled “Adaptive Color Plane Interpolation in Single Sensor Color Electronic Camera,” which is incorporated by reference herein.
U.S. Patent Application Publication No. 2007/0024931, entitled “Image Sensor with Improved Light Sensitivity,” which is incorporated by reference herein, discloses improved CFA patterns, including patterns which provide certain of the pixels with a panchromatic photoresponse. Such patterns are also generally referred to herein as “sparse” CFA patterns. A panchromatic photoresponse has a wider spectral sensitivity than those spectral sensitivities represented in the selected set of color photoresponses and may, for example, have high sensitivity across substantially the entire visible spectrum. Image sensors configured with the improved CFA patterns exhibit greater light sensitivity and are thus well-suited for use in applications involving low scene lighting, short exposure time, small aperture, or other restrictions on the amount of light reaching the image sensor.
In a CCD image sensor, vertical and horizontal CCDs are used to transfer collected charge from the photodiodes or other photosensitive elements of the pixel array. The collected charge from one photosensitive element for a given image capture period is often referred to as a charge packet. A typical CCD image sensor configuration includes a separate vertical CCD (VCCD) for each column of pixels, and at least one horizontal CCD (HCCD) coupled to outputs of the VCCDs. Each VCCD is configured as a shift register having shift elements associated with the respective photosensitive elements of its corresponding column of the pixel array. Outputs of the VCCDs are coupled to respective shift elements of the HCCD. Readout of the pixel array in this type of configuration generally involves shifting charge packets vertically through the VCCDs and into the HCCD, and then shifting the charge packets horizontally through the HCCD to an output amplifier. Often, multi-phase CCD structures are used for the VCCDs and HCCDs in a CCD image sensor. In such an arrangement, shift elements of the VCCD or HCCD are each separated into multiple parts or “phases” to facilitate the transfer of charge packets through these structures.
It is known to combine charge packets from neighboring pixels having the same color filter elements in order to reduce the total number of charge packets. This “color binning” results in reduced image resolution but allows faster image readout. Examples of color binning techniques are disclosed in U.S. Patent Application Publication Nos. 2005/0243195, 2005/0280726, 2006/0044441, 2006/0125943, and 2007/0139545, all of which are commonly assigned herewith and incorporated by reference herein.
Despite the considerable advances provided by the color binning techniques described in the above-cited references, a need remains for finer improvements, particularly in terms of color binning for CCD image sensors having sparse CFA patterns.